1. Field of the Invention
The present invention concerns a reception unit for a magnetic resonance tomography apparatus.
2. Description of the Prior Art
In magnetic resonance tomography apparatus, subjects to be examined (for example body tissue of a patient) are excited to resonate on an atomic level by an excitation unit during a transmission phase. For this purpose, electromagnetic fields of extremely high field strengths are used. In a reception phase subsequent to the transmission phase, the excited regions of the subject to be examined radiate magnetic resonance signals that have field strengths smaller by many orders of magnitude than the excitation signals. The resonance signals are acquired by a reception unit of the magnetic resonance tomography apparatus and converted into image information.
Various local coils receive signals that are amplified by RF preamplifiers and conducted to an evaluation unit to acquire the extremely weak signals during the reception phase. The RF preamplifiers are designed for the extremely small voltages coming from the local coil.
During the transmission phase, instead of the resonance field a local coil is exposed to the electromagnetic excitation field (which as noted above is larger by many orders of magnitude thus the MR field), causing voltages that are higher by orders of magnitude occur at the local coil output. In order to protect the sensitive RF preamplifiers from these voltages, protective diodes (usually PIN diodes) are connected at the amplifier input. During the transmission phase, the PIN diode is fed with current and acts as a short circuit in terms of RF in order to protect the preamplifier from damage and simultaneously to prevent an impermissibly high resonance current from forming in the local coil. Such a resonance current could lead to severe burns to the patient or to unwanted resonance splitting in the magnetic resonance tomography apparatus, i.e. detuning of the individual resonances of the excitation and local coils.
During the reception phase, the PIN diode is supplied with a blocking voltage so that it is non-conductive for the RF signals to be acquired.
The diode current and the diode blocking voltage conventionally have been conducted via separate lines or separate coaxial cable from the external region (field-free region) into the inner region, thus from the site of the local coil or PIN diode into the strong field region of the magnetic resonance tomography apparatus. A problem is that dangerous sheath waves can propagate along all lines that lead from the inside of the magnetic resonance tomography apparatus to the outside. Sheath waves likewise endanger the patient due to skin burns or disrupt the functioning of the preamplifiers due to uncontrolled feedback though the local coil. Therefore, complex sheath wave barriers that prevent the propagation of sheath waves are arranged along all lines leading from the inside of the magnetic resonance tomography apparatus.